1. Field of the Invention
The present invention relates, in general, to a power output apparatus and a method of controlling the same. More specifically, the invention pertains to a power output apparatus for outputting or utilizing power generated by an engine at a high efficiency and a method of controlling such a power output apparatus.
2. Description of the Related Art
In known power output apparatuses for outputting power generated by an engine to a drive shaft, torque converters utilizing a fluid are generally used to convert the power of the engine to a torque transmitted to the drive shaft. In the conventional fluid-based torque converters, an input shaft and an output shaft are not fully locked with each other and there is accordingly an energy loss corresponding to a slip occurring between the input shaft and the output shaft. The energy loss, which is consumed as a heat, is expressed as the product of the revolving speed difference between the input shaft and the output shaft and the torque transmitted at that time. In vehicles with such a power output apparatus mounted thereon, a large energy loss occurs in a transient state like a starting time. The energy efficiency is not 100% even in a stationary driving state. Compared with manual transmissions, the torque converters lead to a lower fuel consumption.
Power output apparatuses for outputting power through mechanical-electrical-mechanical conversion have been proposed to overcome such a drawback (for example, `ARRANGEMENT OF ROTARY POWER-DRIVEN MACHINES` disclosed in JAPANESE PATENT PUBLICATION GAZETTE No. 51-22132). This proposed system connects power generated by an engine with power transmission means including an electromagnetic coupling and a revolving armature, and realizes a reduction ratio (torque conversion ratio) of 1+P2/P1, wherein P1 represents a number of poles of the revolving armature and P2 represents a number of poles of the electromagnetic coupling. Unlike the conventional fluid-based torque converters, this proposed structure substantially has no energy loss due to the slip. It is accordingly possible to make the energy loss in the power transmission means relatively small by enhancing the efficiencies of the electromagnetic coupling and the revolving armature.
The proposed power output apparatus for outputting power through mechanical-electrical-mechanical conversion, however, has a fixed torque conversion ratio and is thus not applicable to the systems requiring a wide variation in torque conversion ratio, such as vehicles. It is difficult to realize a desired torque conversion ratio according to the driving conditions of the vehicle and the engine. As discussed above, power output apparatuses utilizing a liquid can not be free from an energy loss corresponding to a slip occurring between the input shaft and the output shaft.
In engines which rotate the output shaft by pulsating power, for example, internal combustion engines which generate mechanical energy through a cycle of ingestion, compression, combustion, and exhaust, the pulsating power may be output to the drive shaft. When a power output apparatus with such an engine for outputting pulsating power is mounted on a vehicle, the pulsating power output to the drive shaft undesirably vibrates the vehicle itself or the other equipment mounted on the vehicle.
Some apparatuses have been proposed to depress the pulsation of a transmitted torque in the engine rotating the output shaft by its pulsating power. An example of such apparatus is provided with a motor using a flywheel attached to a crankshaft or output shaft of the engine as its rotor, as disclosed in JAPANESE PATENT LAYING-OPEN GAZETTE No. 61-155635. This system enables a torque having an inverted phase to that of the pulsating torque transmitted to the crankshaft to be applied from the motor to the crankshaft via the flywheel, thereby reducing the pulsation of the torque. In this system, however, the flywheel directly attached to the crankshaft of the engine, which is the source of torque pulsation, is used to reduce the torque pulsation. This results in a rather small attenuation of the pulsating component (that is, rather small smoothing effect) and requires a large torque to compensate for the pulsating component.
The pulsation of power affects the behavior of the object to be controlled in the process of controlling operation of the engine or controlling the power transmission means. The minute control detects the behavior due to the power pulsation and thereby implements the unrequired control based on the detected behavior.